Heretofore, stator windings of a distributed winding type or of a concentrated winding type have been considered for rotating electrical machines such as induction motors and synchronous motors. Of these two types, the concentrated winding type is advantageous in that the winding is simpler, the coil end portion can be made smaller, and the ease of mass production can be increased, as compared with the distributed winding type. However, the concentrated winding type suffers in that it is possible that a magnetomotive force harmonic having a frequency close to the fundamental wave may occur in a magnetomotive force waveform, which results in occurrence of torque pulsation (ripple) or a diminished power factor.
For example, FIG. 9 shows a relationship of flux linkage originating from a stator and linked to a rotor with respect to time for an example case in which the stator winding, which is an armature winding of a conventional induction motor, is wound in concentrated winding. Further, FIG. 10 shows a relationship of an induced current occurring in a rotor winding on the side of the rotor, which is a secondary side conductor, corresponding to the flux linkage shown in FIG. 9, with respect to time. As is clear from FIG. 9 and FIG. 10, a spacial harmonic magnetic flux unique to a case wherein a concentrated winding type stator is used is linked to the rotor winding. In other words, the distribution of a magnetomotive force causing a stator to generate a rotating magnetic field does not form a sinusoidal distribution consisting only of a fundamental wave, but includes a harmonic component due to the arrangement of the stator windings of respective phases or the shape of the stator. Particularly, when the stator windings are wound in concentrated winding around teeth on the stator side, the amplitude of a harmonic component occurring in a magnetomotive force distribution of the stator will increase.
A magnetic flux including such a harmonic component due to the arrangement of the stator windings or the shape of the stator is referred to as a spacial harmonic magnetic flux. A harmonic current which is an induced current including a harmonic component as shown in FIG. 10 occurs in the rotor winding to which a spacial harmonic magnetic flux is linked. The harmonic current causes an increase in secondary conductor loss such as secondary copper loss, and deterioration in performance of the motor.
Against this background, Japanese patent publication JP 2006-271187 A (Patent Document 1) discloses a rotating electrical machine comprising a stator in which a coil is wound around a plurality of teeth provided on a stator core in concentrated winding, and the coil is connected to a three-phase power supply; and a rotor disposed to be opposed to the stator, wherein a ratio between the number of poles and the number of slots of the stator is 1:3. Patent Document 1 further describes that there is no magnetomotive force harmonic of an order close to the fundamental wave. In addition to Patent Document 1, prior art documents relevant to the present invention include Japanese patent publications JP 2005-237086 A (Patent Document 2) and JP 2004-56860 A (Patent Document 3).    Patent Document 1: JP 2006-271187 A    Patent Document 2: JP 2005-237086 A    Patent Document 3: JP 2004-56860 A